Like the .WEBP image file, .WEBM is better than mp4 and was created by Google as an efficient means of disseminating media to a large audience. .WEBM video files are relatively small in size, and as such are not as high in terms of quality as some of the other file types on this list. The .WEBM video file format is used for HTML5 video streaming sites, such as YouTube.

https://blog.filestack.com/thoughts-and-knowledge/complete-list-audio-video-file-formats/